Variation in Pubic Symphysis Fusion Across Primates: Implications for Obstetric Adaptation

The unfused human pubic symphysis has been interpreted as an obstetric adaptation to facilitate the passage of a large-brained baby through a relatively small, bipedally adapted pelvis. The degree of fusion of the adult pubic symphysis was evaluated across primate species to gauge whether an open symphysis can be interpreted as an obstetric adaptation in humans and other primates.

Symphyseal fusion was assessed in 718 individuals from 67 nonhuman primate species. Variation in fusion in specimens of known ages and sex from four species (Galago moholi, Macaca mulatta, Microcebus murinus, and Pan troglodytes) was further examined, with detailed analyses of pubic changes by age and sex carried out through logistic regressions in macaques.

Pubic fusion occurs in most primate species. It is observed earlier in life in males than in females in Ma. mulatta and Pa. troglodytes, only in males in Mi. murinus, and does not occur in Ga. moholi.

While delayed or absent pubic fusion is more prevalent in female primates, suggesting obstetric adaptation, there is no clear relation with childbirth constraints, as fusion is also observed in species experiencing a tight cephalopelvic fit. Other mechanisms might have evolved to facilitate birth in some species, or nonobstetric selective pressures might be counteracting the obstetric advantages of a flexible symphysis. The preservation of an open symphysis throughout life in humans and some other primates, however, can be best interpreted as convergent evolution due to obstetric selection.